Real and imaginary parts of the dielectric function of TlIn(S(1-x)Sex)(2) (x = 0.05, 0.08, 0.25) single crystals were determined in the spectral range from 1 to 5 eV within a temperature interval 140-293 K from spectroscopic ellipsometry measurements. The energies of interband transitions (critical points) of the TlIn(S(1-x)Sex)(2) crystals were obtained from the second derivative of the real and imaginary parts of dielectric function. Structural phase transitions are behind the observed change of electronic band structure. (C) 2017 Elsevier B.V. All rights reserved.